PETROPHYSICAL AND GEOMECHANICAL ROCK PROPERTIES: EXAMPLE FROM JURASSIC LIMESTONE AND DOLOSTONE, PELORITANI MOUNTS (NORTH-EASTERN SICILY, ITALY)

This work deals with the relationship between textural, petrophysical and geomechanical properties of carbonate rocks. To this aim, a suite of representative limestone and dolostone cropping out in the southern sector of the Peloritani Mounts (north-west Sicily, Italy) has been selected. These lithotypes, which constitute the Jurassic sedimentary cover of the Hercynian crystalline basement, have been ascribed to the Taormina Unit (Lentini, 1973). The geomechanical investigation highlighted similar average density values of 2,80 g/cm3 (limestone) and 2,85 g/cm3 (dolostone), respectively; these relatively high values have been explained with the good compactness of the studied rock-types, characterised by low porosity. Moreover, in situ measurements of Uniaxial Compressive Strength (UCS) with the Schmidt hammer average UCS (according to Deere and Miller, 1966) showed similar values of 44.40MPa (Stand. Dev. 4.91) in limestone and 45.17MPa (Stand Dev. 7.87) in dolostone. On the whole, the higher values of the standard deviation in dolostone have been interpreted as the consequence of the occurrence of microfracture systems. Differently, the Point Load Test (PLT) investigation carried out in laboratory according to ISRM (1985), whose results were calculated after Broch and Franklin (1972), put into evidence different average values, which are 40.32MPa in limestone and 74.40 in dolostone. PLT results have been also used for calculating the Strength Anisotropy Index Ia(50) according to ISRM (1985). Referred to this index, both lithotypes resulted to be anisotropic since Ia(50)>1, and in particular 4.80 in limestone and 6.07 in dolostone. Aimed to the petrophysical investigation, samples were cut into 50 mm edged cubes and ultrasound velocity measurements were carried out along three orthogonal directions at room P-T conditions, according to the ASTM (designation 2845-00) recommendation. The mean ultrasound velocity values are 6,65 km/s for limestone and 6,27 km/s as far as dolostone. In addition, velocity measurements along the three directions permitted the seismic anisotropy to de determined, where A%= (Vmax-Vmin)/Vmean (Birch, 1961). Results pointed out that within each lithotype group there are evident variations in anisotropy values (1,34-4,45% in limestone and 0,97-7,74% in dolostone, respectively). Provided that the studied rocks may be considered as mono mineralic –polycrystalline aggregates, textural investigation on thin sections evidenced different petrofabric features for the two lithotypes. Indeed, limestone is characterised by irregular and curved inter-crystalline calcite boundaries (xenotopic texture), being the former pores entirely filled with sparite cement; this would suggest that an enhanced diagenetic process involved changes in fabric. Differently, within dolostone the dolomite grains are arranged in an iditopic texture, with open pores occurring. These textural features may be strongly related to the petrophysical and geo-mechanical behaviour of the two lithotypes investigated